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Chapter 12 Theory Introduction Weather is an important factor that influences aircraft performance and flying safety. It is the state of the at a given time and place with respect to variables, such as ( or cold), moisture (wetness or dryness), velocity (calm or ), (clearness or cloudiness), and barometric (high or low). The term “weather” can also apply to adverse or destructive atmospheric conditions, such as high .

This chapter explains basic weather theory and offers pilots background knowledge of weather principles. It is designed to help them gain a good understanding of how weather affects daily flying activities. Understanding the theories behind weather helps a pilot make weather decisions based on the reports and forecasts obtained from a Service Station (FSS) weather specialist and other aviation weather services.

Be it a local flight or a long cross-country flight, decisions based on weather can dramatically affect the safety of the flight.

12-1 Atmosphere The atmosphere is a blanket of air made up of a of 1 gases that surrounds the and reaches almost 350 miles from the surface of the Earth. This mixture is in constant motion. If the atmosphere were visible, it might look like 2211 an with swirls and eddies, rising and falling air, and Oygen waves that travel for great distances. Life on Earth is supported by the atmosphere, solar , 77 and the ’s magnetic fields. The atmosphere absorbs 88 energy from the , recycles and other chemicals, and itrogen works with the electrical and magnetic forces to provide a moderate . The atmosphere also protects life on Earth from high energy and the frigid of space. Figure 12-1. Composition of the atmosphere. Composition of the Atmosphere In any given volume of air, accounts for 78 percent atmosphere have been identified using characteristics of the gases that comprise the atmosphere, while (temperature changes), chemical composition, movement, makes up 21 percent. , dioxide, and traces and . [Figure 12-2] of other gases make up the remaining one percent. This volume of air also contains some water , varying from The first layer, known as the , extends from 6 zero to about five percent by volume. This small amount of to 20 kilometers (km) (4 to 12 miles) over the northern and is responsible for major changes in the weather. southern poles and up to 48,000 feet (14.5 km) over the [Figure 12-1] equatorial regions. The vast majority of weather, , , and temperature variances occur within this first The envelope of gases surrounding the Earth changes layer of the atmosphere. Inside the troposphere, the average from the up. Four distinct layers or spheres of the temperature decreases at a rate of about 2 ° (C) every

e her 000 feet osp 280 erm Th re feet he 160000 osp es M ere 00 feet 200 os rat St ere ph os

op Tr

Figure 12-2. Layers of the atmosphere.

12-2 1,000 feet of gain, and the pressure decreases at a rate of about one inch per 1,000 feet of altitude gain.

At the top of the troposphere is a boundary known as the , which traps moisture and the associated weather in the troposphere. The altitude of the tropopause varies with and with the of the year; therefore, it takes on an elliptical shape as opposed to round. Location of the tropopause is important because it is commonly associated with the location of the jet and possible clear air .

Above the tropopause are three more atmospheric levels. The first is the , which extends from the tropopause to a of about 160,000 feet (50 km). Little weather exists in this layer and the air remains stable, although certain types of clouds occasionally extend in it. Above the stratosphere are the and , which have little influence over weather. Figure 12-3. Circulation pattern in a static environment.

Atmospheric Circulation As noted earlier, the atmosphere is in constant motion. The unequal heating of the Earth’s surface not only modifies Certain factors combine to set the atmosphere in motion, but a air density and creates circulation patterns; it also causes major factor is the uneven heating of the Earth’s surface. This changes in air pressure or the force exerted by the weight heating upsets the equilibrium of the atmosphere, creating of air molecules. Although air molecules are invisible, they changes in air movement and atmospheric pressure. The still have weight and take up space. movement of air around the surface of the Earth is called . Imagine a sealed column of air that has a footprint of one square inch and is 350 miles high. It would take 14.7 pounds Heating of the Earth’s surface is accomplished by several of effort to lift that column. This represents the air’s weight; processes, but in the simple -only model used for if the column is shortened, the pressure exerted at the bottom this discussion, the Earth is warmed by energy radiating from (and its weight) would be less. the sun. The process causes a circular motion that results when warm air rises and is replaced by cooler air. The weight of the shortened column of air at 18,000 feet is approximately 7.4 pounds; almost 50 percent that at . Warm air rises because heat causes air molecules to spread For instance, if a bathroom scale (calibrated for sea level) apart. As the air expands, it becomes less dense and lighter were raised to 18,000 feet, the column of air weighing 14.7 than the surrounding air. As air cools, the molecules pack pounds at sea level would be 18,000 feet shorter and would together more closely, becoming denser and heavier than weigh approximately 7.3 pounds (50 percent) less than at warm air. As a result, cool, heavy air tends to sink and replace sea level. [Figure 12-4] warmer, rising air. The actual pressure at a given place and time differs with Because the Earth has a curved surface that rotates on a tilted altitude, temperature, and density of the air. These conditions axis while orbiting the sun, the equatorial regions of the Earth also affect aircraft performance, especially with regard to receive a greater amount of heat from the sun than the polar takeoff, rate of climb, and landings. regions. The amount of that the Earth depends on the time of year and the latitude of the specific region. All of these factors affect the length of time and the In general atmospheric circulation theory, areas of low angle at which strikes the surface. pressure exist over the equatorial regions and areas of high pressure exist over the polar regions due to a difference in Solar heating causes higher in equatorial areas, temperature. The resulting low pressure allows the high- which causes the air to be less dense and rise. As the warm pressure air at the poles to flow along the planet’s surface air flows toward the poles, it cools, becoming denser and toward the . While this pattern of air circulation is sinks back toward the surface. [Figure 12-3]

12-3 1 Suare Inch 1 Suare Inch

1

1 Suare Inch 1 Suare Inch 7.4 lb 1800018000 feetfeet

1

14.7 Sea level lb Figure 12-5. Three-cell circulation pattern due to the rotation of the Earth.

Figure 12-4. Atmosphere weights. the surface. Then, it flows southward along the surface back toward the equator. Coriolis force bends the flow to the right, correct in theory, the circulation of air is modified by several thus creating the northeasterly that prevail from forces, the most important of which is the rotation of the Earth. 30° latitude to the equator. Similar forces create circulation cells that encircle the Earth between 30° and 60° latitude and The force created by the rotation of the Earth is known as between 60° and the poles. This circulation pattern results in the Coriolis force. This force is not perceptible to humans as the prevailing upper level westerly winds in the conterminous they walk around because humans move slowly and travel . relatively short distances compared to the size and rotation rate of the Earth. However, the Coriolis force significantly Circulation patterns are further complicated by seasonal affects motion over large distances, such as an or changes, differences between the surfaces of and body of water. , and other factors such as frictional forces caused by the topography of the Earth’s surface that modify the The Coriolis force deflects air to the right in the Northern movement of the air in the atmosphere. For example, within Hemisphere, causing it to follow a curved path instead of a 2,000 feet of the ground, the friction between the surface and straight line. The amount of deflection differs depending on the atmosphere slows the moving air. The wind is diverted the latitude. It is greatest at the poles and diminishes to zero from its path because of the frictional force. Thus, the wind at the equator. The magnitude of Coriolis force also differs direction at the surface varies somewhat from the wind with the speed of the moving body—the greater the speed, direction just a few thousand feet above the Earth. the greater the deviation. In the , the rotation of the Earth deflects moving air to the right and Measurement of Atmosphere Pressure changes the general circulation pattern of the air. Atmospheric pressure historically was measured in inches of ("Hg) by a mercurial . [Figure 12-6] The The Coriolis force causes the general flow to break up into barometer measures the height of a column of mercury inside a three distinct cells in each hemisphere. [Figure 12-5] In tube. A section of the mercury is exposed to the pressure the Northern Hemisphere, the warm air at the equator rises of the atmosphere, which exerts a force on the mercury. An upward from the surface, travels northward, and is deflected increase in pressure forces the mercury to rise inside the tube. eastward by the rotation of the Earth. By the time it has When the pressure drops, mercury drains out of the tube traveled one-third of the distance from the equator to the decreasing the height of the column. This type of barometer is North Pole, it is no longer moving northward, but eastward. typically used in a laboratory or weather observation station, This air cools and sinks in a belt-like area at about 30° is not easily transported, and difficult to read. latitude, creating an area of high pressure as it sinks toward

12-4 the linkage mechanism of an aneroid barometer, it is not as accurate as a mercurial barometer. 147 2 To provide a common reference, the International Standard Atmosphere (ISA) has been established. These standard 2992 conditions are the basis for certain flight instruments and most aircraft performance data. Standard sea level pressure is defined as 29.92 "Hg and a standard temperature of 59 °F (15 °C). Atmospheric pressure is also reported in millibars (mb), with 1 "Hg equal to approximately 34 mb. Standard sea

760 mm level pressure is 1,013.2 mb. Typical mb pressure readings range from 950.0 to 1,040.0 mb. Surface charts, high and low pressure centers, and hurricane data are reported using mb. Height of mercury 29.92 "

Since weather stations are located around the globe, all local barometric pressure readings are converted to a sea level pressure to provide a standard for records and reports. To achieve this, each station converts its barometric pressure by adding approximately 1 "Hg for every 1,000 feet of . For example, a station at 5,000 feet above sea level, with a reading of 24.92 "Hg, reports a sea level pressure reading of 29.92 "Hg. [Figure 12-8] Using common sea level pressure 2992 " 10132 147 2 readings helps ensure aircraft are set correctly, based on the current pressure readings. Figure 12-6. Although mercurial are no longer used in the U. S., they are still a good historical reference for where the By tracking barometric pressure trends across a large area, setting came from (inches of mercury). weather forecasters can more accurately predict movement of pressure systems and the associated weather. For example, An aneroid barometer is the standard instrument used tracking a pattern of rising pressure at a single to measure pressure; it is easier to read and . generally indicates the approach of fair weather. Conversely, [Figure 12-7] The aneroid barometer contains a closed vessel decreasing or rapidly falling pressure usually indicates called an aneroid cell that contracts or expands with changes approaching bad weather and, possibly, severe storms. in pressure. The aneroid cell attaches to a pressure indicator with a mechanical linkage to provide pressure readings. The Altitude and Atmospheric Pressure pressure sensing part of an aircraft altimeter is essentially As altitude increases, atmospheric pressure decreases. On an aneroid barometer. It is important to note that due to average, with every 1,000 feet of increase in altitude, the atmospheric pressure decreases 1 "Hg. As pressure decreases, the air becomes less dense or thinner. This is the equivalent of Higher being at a higher altitude and is referred to as . As pressure decreases, density altitude increases and has a pronounced effect on aircraft performance.

Differences in air density caused by changes in temperature result in a change in pressure. This, in turn, creates motion in the atmosphere, both vertically and horizontally, in the form Lower Sealed aneroid cell of currents and wind. The atmosphere is almost constantly in motion as it strives to reach equilibrium. These never-ending Sealed aneroid cell air movements set up chain reactions that cause a continuing Sealed aneroid cell variety in the weather.

Figure 12-7. Aneroid barometer.

12-5 24.92 "Hg

29.92 "Hg

2992 2992

Figure 12-8. Station pressure is converted to and reported in sea level pressure.

Altitude and Flight is longer. An aircraft that requires 745 feet of ground run at sea level requires more than double that at a Altitude affects every aspect of flight from aircraft of 8,000 feet. [Figure 12-9]. It is also true that at higher performance to human performance. At higher , altitudes, due to the decreased density of the air, aircraft with a decreased atmospheric pressure, takeoff and landing engines and propellers are less efficient. This leads to reduced distances are increased, while climb rates decrease. rates of climb and a greater ground run for obstacle clearance. When an aircraft takes off, lift is created by the flow of air around the wings. If the air is thin, more speed is required Altitude and the Human Body to obtain enough lift for takeoff; therefore, the ground run As discussed earlier, nitrogen and other gases make up 79 percent of the atmosphere, while the remaining 21

TAEOFF DISTACE 2400

0 50

745 1320 1000 815 1445 2000 895 1585 3000 980 1740 745 4000 1075 1920 5000 1185 2125 6000 1305 2360 7000 1440 2635 8000 1590 2960 8000

1590

Figure 12-9. Takeoff distances increase with increased altitude.

12-6 percent is life sustaining atmospheric oxygen. At sea level, atmospheric pressure is great enough to support normal growth, activity, and life. By 18,000 feet, the partial pressure of oxygen is reduced and adversely affects the normal activities and functions of the human body.

The reactions of the average person become impaired at an altitude of about 10,000 feet, but for some people impairment can occur at an altitude as low as 5,000 feet. The physiological reactions to or oxygen deprivation are insidious and affect people in different ways. These symptoms range from mild disorientation to total incapacitation, depending on body tolerance and altitude. Supplemental oxygen or cabin pressurization systems help pilots fly at higher altitudes and overcome the effects of oxygen deprivation.

Wind and Currents Air flows from areas of high pressure into areas of low pressure because air always seeks out lower pressure. The Figure 12-10. Circulation pattern about areas of high and low combination of atmospheric pressure differences, Coriolis pressure. force, friction, and temperature differences of the air near the earth cause two kinds of atmospheric motion: convective is a better understanding of what type of weather to expect currents (upward and downward motion) and wind in a given area along a route of flight based on the prevailing (horizontal motion). Currents and winds are important as areas of highs and lows. they affect takeoff, landing, and cruise flight operations. Most importantly, currents and winds or atmospheric circulation While the theory of circulation and wind patterns is accurate for cause weather changes. large scale atmospheric circulation, it does not take into account changes to the circulation on a local scale. Local conditions, Wind Patterns geological features, and other anomalies can change the wind In the Northern Hemisphere, the flow of air from areas of direction and speed close to the Earth’s surface. high to low pressure is deflected to the right and produces a clockwise circulation around an area of high pressure. Convective Currents This is known as anticyclonic circulation. The opposite Plowed ground, rocks, sand, and barren land absorb solar is true of low-pressure areas; the air flows toward a low energy quickly and can therefore give off a large amount and is deflected to create a counterclockwise or cyclonic of heat; whereas, water, trees, and other areas of vegetation circulation. [Figure 12-10] tend to more slowly absorb heat and give off heat. The resulting uneven heating of the air creates small areas of High-pressure systems are generally areas of dry, descending local circulation called convective currents. air. Good weather is typically associated with high-pressure systems for this reason. Conversely, air flows into a low- Convective currents cause the bumpy, turbulent air sometimes pressure area to replace rising air. This air usually brings experienced when flying at lower altitudes during warmer increasing cloudiness and . Thus, bad weather weather. On a low-altitude flight over varying surfaces, is commonly associated with areas of low pressure. updrafts are likely to occur over pavement or barren places, and downdrafts often occur over water or expansive areas A good understanding of high- and low-pressure wind patterns of vegetation like a group of trees. Typically, these turbulent can be of great help when planning a flight because a pilot can conditions can be avoided by flying at higher altitudes, even take advantage of beneficial tailwinds. [Figure 12-11] When above cumulus layers. [Figure 12-12] planning a flight from west to east, favorable winds would be encountered along the northern side of a high-pressure Convective currents are particularly noticeable in areas with a system or the southern side of a low-. On land mass directly adjacent to a large body of water, such as an the return flight, the most favorable winds would be along ocean, large lake, or other appreciable area of water. During the southern side of the same high-pressure system or the the day, land heats faster than water, so the air over the land northern side of a low-pressure system. An added advantage becomes warmer and less dense. It rises and is replaced by

12-7 Figure 12-11. Favorable winds near a high pressure system.

Figure 12-12. Convective turbulence avoidance. cooler, denser air flowing in from over the water. This causes Effect of Obstructions on Wind an onshore wind called a . Conversely, at land Another atmospheric hazard exists that can create problems cools faster than water, as does the corresponding air. In this for pilots. Obstructions on the ground affect the flow of case, the warmer air over the water rises and is replaced by wind and can be an unseen danger. Ground topography and the cooler, denser air from the land, creating an offshore wind large buildings can break up the flow of the wind and create called a land breeze. This reverses the local wind circulation wind gusts that change rapidly in direction and speed. These pattern. Convective currents can occur anywhere there is an obstructions range from man-made structures, like hangars, uneven heating of the Earth’s surface. [Figure 12-13] to large natural obstructions, such as , bluffs, or canyons. It is especially important to be vigilant when flying Convective currents close to the ground can affect a pilot’s in or out of that have large buildings or natural ability to control the aircraft. For example, on final approach, obstructions located near the runway. [Figure 12-15] the rising air from terrain devoid of vegetation sometimes produces a ballooning effect that can cause a pilot to overshoot The intensity of the turbulence associated with ground the intended landing spot. On the other hand, an approach over obstructions depends on the size of the obstacle and the a large body of water or an area of thick vegetation tends to primary velocity of the wind. This can affect the takeoff and create a sinking effect that can cause an unwary pilot to land landing performance of any aircraft and can present a very short of the intended landing spot. [Figure 12-14] serious hazard. During the landing phase of flight, an aircraft

12-8 Cool Warm

Warm Cool

Figure 12-13. Sea breeze and land breeze wind circulation patterns.

Cool sining air

Warm rising air

Figure 12-14. Currents generated by varying surface conditions.

12-9

W I N D

Figure 12-15. Turbulence caused by manmade obstructions. may “drop in” due to the turbulent air and be too low to clear a similar manner. As the air flows down the leeward side of obstacles during the approach. the , the air follows the contour of the terrain and is increasingly turbulent. This tends to push an aircraft into This same condition is even more noticeable when flying in the side of a mountain. The stronger the wind, the greater the mountainous regions. [Figure 12-16] While the wind flows downward pressure and turbulence become. smoothly up the windward side of the mountain and the upward currents help to carry an aircraft over the peak of Due to the effect terrain has on the wind in valleys or canyons, the mountain, the wind on the leeward side does not act in downdrafts can be severe. Before conducting a flight in or

WID

Figure 12-16. Turbulence in mountainous regions.

12-10 near mountainous terrain, it is helpful for a pilot unfamiliar and headwind losses of 30–90 knots, seriously degrading with a mountainous area to get a checkout with a mountain performance. It can also produce strong turbulence and qualified flight instructor. hazardous changes. Consider Figure 12-17: During an inadvertent takeoff into a microburst, the plane Low-Level may first experience a performance-increasing headwind Wind shear is a sudden, drastic change in and/or (1), followed by performance-decreasing downdrafts (2), direction over a very small area. Wind shear can subject an followed by a rapidly increasing tailwind (3). This can result aircraft to violent updrafts and downdrafts, as well as abrupt in terrain impact or flight dangerously close to the ground (4). changes to the horizontal movement of the aircraft. While An encounter during approach involves the same sequence wind shear can occur at any altitude, low-level wind shear is of wind changes and could force the plane to the ground especially hazardous due to the proximity of an aircraft to the short of the runway. ground. Low-level wind shear is commonly associated with passing frontal systems, , temperature inversions, The FAA has made a substantial investment in microburst and strong upper level winds (greater than 25 knots). prevention. The totally redesigned LLWAS-NE, the TDWR, and the ASR-9 WSP are skillful microburst alerting Wind shear is dangerous to an aircraft. It can rapidly change systems installed at major airports. These three systems were the performance of the aircraft and disrupt the normal flight extensively evaluated over a 3-year period. Each was seen attitude. For example, a tailwind quickly changing to a to issue very few false alerts and to detect microbursts well headwind causes an increase in airspeed and performance. above the 90 percent detection requirement established by Conversely, a headwind changing to a tailwind causes a Congress. Many involve airports that lack microburst decrease in airspeed and performance. In either case, a pilot alert equipment, so the FAA has also prepared wind shear must be prepared to react immediately to these changes to training material: Advisory Circular (AC) 00-54, FAA maintain control of the aircraft. Pilot Wind Shear Guide. Included is information on how to recognize the risk of a microburst encounter, how to avoid an The most severe type of low-level wind shear, a microburst, encounter, and the best flight strategy for successful escape is associated with convective precipitation into dry air at should an encounter occur. . Microburst activity may be indicated by an intense shaft at the surface but at cloud base It is important to remember that wind shear can affect any and a ring of blowing is often the only visible clue. flight and any pilot at any altitude. While wind shear may be A typical microburst has a horizontal diameter of 1–2 reported, it often remains undetected and is a silent danger miles and a nominal depth of 1,000 feet. The lifespan of a to aviation. Always be alert to the possibility of wind shear, microburst is about 5–15 minutes during which time it can especially when flying in and around thunderstorms and produce downdrafts of up to 6,000 feet per minute (fpm) frontal systems.

Strong downdraft

Intended Path

ind ailw I In ing t nc creas re asi ng headwind 2

Outflow 4 1 3

Figure 12-17. Effects of a microburst wind.

12-11 Wind and Pressure Representation on Surface Weather Maps 1028 Surface weather maps provide information about fronts, areas of high and low pressure, and surface winds and for each station. This type of allows pilots to see the locations of fronts and pressure systems, but more 1024 importantly, it depicts the wind and pressure at the surface for each location. For more information on surface analysis and weather depiction charts, see Chapter 13, Aviation 1020 Weather Services.

Wind conditions are reported by an arrow attached to the 1016 station location circle. [Figure 12-18] The station circle represents the head of the arrow, with the arrow pointing in the direction from which the wind is blowing. Winds are described by the direction from which they blow, thus a northwest wind means that the wind is blowing from the 1012 northwest toward the southeast. The speed of the wind is depicted by barbs or pennants placed on the wind line. Each 1008 L barb represents a speed of ten knots, while half a barb is equal to five knots, and a pennant is equal to 50 knots. Figure 12-19. Isobars reveal the of an area of high- or low-pressure areas. The pressure for each station is recorded on the weather chart and is shown in mb. Isobars are lines drawn on the chart to wind direction is modified by the friction and wind speed depict lines of equal pressure. These lines result in a pattern decreases due to friction with the surface. At levels 2,000 to that reveals the pressure gradient or change in pressure over 3,000 feet above the surface, however, the speed is greater distance. [Figure 12-19] Isobars are similar to contour lines and the direction becomes more parallel to the isobars. on a topographic map that indicate terrain altitudes and slope steepness. For example, isobars that are closely spaced Generally, the wind 2,000 feet above ground level (AGL) is indicate a steep pressure gradient and strong winds prevail. 20° to 40° to the right of surface winds, and the wind speed is Shallow gradients, on the other hand, are represented by greater. The change of wind direction is greatest over rough isobars that are spaced far apart and are indicative of terrain and least over flat surfaces, such as open water. In the winds. Isobars help identify low- and high-pressure systems, absence of winds aloft information, this rule of thumb allows as well as the location of ridges and troughs. A high is an for a rough estimate of the wind conditions a few thousand area of high pressure surrounded by lower pressure; a low feet above the surface. is an area of low pressure surrounded by higher pressure. A is an elongated area of high pressure, and a is Atmospheric Stability an elongated area of low pressure. The stability of the atmosphere depends on its ability to resist vertical motion. A stable atmosphere makes vertical Isobars furnish valuable information about winds in the first movement difficult, and small vertical disturbances dampen few thousand feet above the surface. Close to the ground, out and disappear. In an unstable atmosphere, small vertical air movements tend to become larger, resulting in turbulent airflow 5 20 and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and .

Rising air expands and cools due to the decrease in air pressure as altitude increases. The opposite is true of 35 50 105 descending air; as atmospheric pressure increases, the temperature of descending air increases as it is compressed. Adiabatic heating and adiabatic cooling are terms used to describe this temperature change.

Figure 12-18. Depiction of winds on a surface weather chart.

12-12 The takes place in all upward and Moisture and Temperature downward moving air. When air rises into an area of lower The atmosphere, by , contains moisture in the form pressure, it expands to a larger volume. As the molecules of water vapor. The amount of moisture present in the of air expand, the temperature of the air lowers. As a result, atmosphere is dependent upon the temperature of the air. when a parcel of air rises, pressure decreases, volume Every 20 °F increase in temperature doubles the amount of increases, and temperature decreases. When air descends, moisture the air can hold. Conversely, a decrease of 20 °F the opposite is true. The rate at which temperature decreases cuts the capacity in half. with an increase in altitude is referred to as its . As air ascends through the atmosphere, the average rate of Water is present in the atmosphere in three states: , temperature change is 2 °C (3.5 °F) per 1,000 feet. , and gaseous. All three forms can readily change to another, and all are present within the temperature ranges of Since water vapor is lighter than air, moisture decreases air the atmosphere. As water changes from one state to another, density, causing it to rise. Conversely, as moisture decreases, an exchange of heat takes place. These changes occur through air becomes denser and tends to sink. Since moist air cools the processes of , sublimation, , at a slower rate, it is generally less stable than dry air since deposition, , or . However, water vapor the moist air must rise higher before its temperature cools is added into the atmosphere only by the processes of to that of the surrounding air. The dry adiabatic lapse rate evaporation and sublimation. (unsaturated air) is 3 °C (5.4 °F) per 1,000 feet. The moist adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to Evaporation is the changing of liquid water to water vapor. 5 °F) per 1,000 feet. As water vapor forms, it absorbs heat from the nearest available source. This heat exchange is known as the latent The combination of moisture and temperature determine the heat of evaporation. A good example is the evaporation of stability of the air and the resulting weather. Cool, dry air human . The net effect is a cooling sensation is very stable and resists vertical movement, which leads to as heat is extracted from the body. Similarly, sublimation good and generally clear weather. The greatest instability is the changing of directly to water vapor, completely occurs when the air is moist and warm, as it is in the tropical bypassing the liquid stage. Though is not made of regions in the . Typically, thunderstorms appear on water, but rather , it demonstrates the principle a daily basis in these regions due to the instability of the of sublimation when a solid turns directly into vapor. surrounding air. Relative Humidity refers to the amount of water vapor present in the As air rises and expands in the atmosphere, the temperature atmosphere at a given time. Relative humidity is the actual decreases. There is an atmospheric anomaly that can occur; amount of moisture in the air compared to the total amount of however, that changes this typical pattern of atmospheric moisture the air could hold at that temperature. For example, behavior. When the temperature of the air rises with altitude, a if the current relative humidity is 65 percent, the air is temperature inversion exists. Inversion layers are commonly holding 65 percent of the total amount of moisture that it is shallow layers of smooth, stable air close to the ground. The capable of holding at that temperature and pressure. While temperature of the air increases with altitude to a certain much of the western United States rarely sees days of high point, which is the top of the inversion. The air at the top humidity, relative humidity readings of 75 to 90 percent are of the layer acts as a lid, keeping weather and not uncommon in the during warmer trapped below. If the relative humidity of the air is high, it months. [Figure 12-20] can contribute to the formation of clouds, , , or resulting in diminished visibility in the inversion layer. Temperature/ Point Relationship The relationship between and temperature defines Surface-based temperature inversions occur on clear, cool the concept of relative humidity. The dew point, given in when the air close to the ground is cooled by the degrees, is the temperature at which the air can hold no lowering temperature of the ground. The air within a few more moisture. When the temperature of the air is reduced hundred feet of the surface becomes cooler than the air above to the dew point, the air is completely saturated and moisture it. Frontal inversions occur when warm air spreads over a begins to condense out of the air in the form of fog, dew, layer of cooler air, or cooler air is forced under a layer of , clouds, rain, or . warmer air.

12-13 At sea level pressure air can hold

9 2 10 17 2 20 30 2 30

10 9 8 100

30 17 56 17 56 30

17 20 100 20

Figure 12-20. Relationship between relative humidity, temperature, and dewpoint.

As moist, unstable air rises, clouds often form at the altitude Explanation: where temperature and dew point reach the same value. When With an outside air temperature (OAT) of 85 °F at the surface lifted, unsaturated air cools at a rate of 5.4 °F per 1,000 feet and dew point at the surface of 71 °F, the spread is 14°. Divide and the dew point temperature decreases at a rate of 1 °F per the temperature dew point spread by the convergence rate of 1,000 feet. This results in a convergence of temperature and 4.4 °F, and multiply by 1,000 to determine the approximate dew point at a rate of 4.4 °F. Apply the convergence rate height of the cloud base. to the reported temperature and dew point to determine the height of the cloud base. Methods by Which Air Reaches the Saturation Point If air reaches the saturation point while temperature and Given: dew point are close together, it is highly likely that fog, low Temperature (T) = 85 °F clouds, and precipitation will form. There are four methods by which air can reach the saturation point. First, when warm Dew point (DP) = 71 °F air moves over a cold surface, the air temperature drops and Convergence Rate (CR) = 4.4° reaches the saturation point. Second, the saturation point may T – DP = Temperature Dew Point Spread (TDS) be reached when cold air and warm air mix. Third, when air cools at night through contact with the cooler ground, air TDS ÷ CR = X reaches its saturation point. The fourth method occurs when X × 1,000 feet = height of cloud base AGL air is lifted or is forced upward in the atmosphere. Example: As air rises, it uses heat energy to expand. As a result, the rising 85 °F – 71 °F = 14 °F air loses heat rapidly. Unsaturated air loses heat at a rate of 14 °F ÷ 4.4 °F = 3.18 3.0 °C (5.4 °F) for every 1,000 feet of altitude gain. No 3.18 × 1,000 = 3,180 feet AGL what causes the air to reach its saturation point, saturated air brings clouds, rain, and other critical weather situations. The height of the cloud base is 3,180 feet AGL.

12-14 Dew and Frost allow the fog to form and intensify; above a speed of 15 knots, On cool, clear, calm nights, the temperature of the ground the fog usually lifts and forms low stratus clouds. and objects on the surface can cause temperatures of the fog is common in coastal areas where sea breezes can blow surrounding air to drop below the dew point. When this the air over cooler landmasses. occurs, the moisture in the air condenses and deposits itself on the ground, buildings, and other objects like cars and aircraft. Upslope fog occurs when moist, stable air is forced up sloping This moisture is known as dew and sometimes can be seen land features like a mountain range. This type of fog also on grass and other objects in the morning. If the temperature requires wind for formation and continued existence. Upslope is below freezing, the moisture is deposited in the form of and advection fog, unlike radiation fog, may not burn off with frost. While dew poses no threat to an aircraft, frost poses a the morning sun but instead can persist for days. They can definite flight safety hazard. Frost disrupts the flow of air over also extend to greater than radiation fog. the wing and can drastically reduce the production of lift. It also increases drag, which when combined with lowered lift fog, or sea smoke, forms when cold, dry air moves over production, can adversely affect the ability to take off. An warm water. As the water evaporates, it rises and resembles aircraft must be thoroughly cleaned and free of frost prior smoke. This type of fog is common over bodies of water to beginning a flight. during the coldest times of the year. Low-level turbulence and icing are commonly associated with steam fog. Fog Fog is a cloud that is on the surface. It typically occurs when occurs in cold weather when the temperature is the temperature of air near the ground is cooled to the air’s much below freezing and water vapor forms directly into ice dew point. At this point, water vapor in the air condenses and . Conditions favorable for its formation are the same becomes visible in the form of fog. Fog is classified according as for radiation fog except for cold temperature, usually –25 to the manner in which it forms and is dependent upon the °F or colder. It occurs mostly in the regions but is not current temperature and the amount of water vapor in the air. unknown in middle during the cold season.

On clear nights, with relatively little to no wind present, Clouds radiation fog may develop. [Figure 12-21] Usually, it forms Clouds are visible indicators and are often indicative of in low-lying areas like mountain valleys. This type of fog future weather. For clouds to form, there must be adequate occurs when the ground cools rapidly due to terrestrial water vapor and condensation nuclei, as well as a method by radiation, and the surrounding air temperature reaches its which the air can be cooled. When the air cools and reaches dew point. As the sun rises and the temperature increases, its saturation point, the invisible water vapor changes into radiation fog lifts and eventually burns off. Any increase in a visible state. Through the processes of deposition (also wind also speeds the of radiation fog. If radiation referred to as sublimation) and condensation, moisture fog is less than 20 feet thick, it is known as ground fog. condenses or sublimates onto miniscule particles of matter like dust, , and smoke known as condensation nuclei. The When a layer of warm, moist air moves over a cold surface, nuclei are important because they provide a means for the advection fog is likely to occur. Unlike radiation fog, wind moisture to change from one state to another. is required to form advection fog. Winds of up to 15 knots Cloud type is determined by its height, shape, and characteristics. They are classified according to the height of their bases as low, middle, or high clouds, as well as clouds with vertical development. [Figure 12-22]

Low clouds are those that form near the Earth’s surface and extend up to about 6,500 feet AGL. They are made primarily of water droplets but can include supercooled water droplets that induce hazardous aircraft icing. Typical low clouds are stratus, stratocumulus, and nimbostratus. Fog is also classified as a type of low cloud formation. Clouds in this family create low ceilings, hamper visibility, and can change rapidly. Because of this, they influence flight planning and can make visual flight rules (VFR) flight impossible. Figure 12-21. Radiation fog.

12-15 Cirrocumulus Cirrostratus

Cumulonimbus Cirrus High clouds

20000 AL

Altostratus Altocumulus Middle clouds

6500 AL

Stratocumulus Towering Cumulus Stratus imbostratus Low clouds

Figure 12-22. Basic cloud types.

Middle clouds form around 6,500 feet AGL and extend up to and usually produce hazardous weather phenomena, such 20,000 feet AGL. They are composed of water, , as , , tornadoes, gusty winds, and wind shear. and supercooled water droplets. Typical middle-level clouds These extensive vertical clouds can be obscured by other include altostratus and altocumulus. These types of clouds cloud formations and are not always visible from the ground may be encountered on cross-country flights at higher or while in flight. When this happens, these clouds are said altitudes. Altostratus clouds can produce turbulence and may to be embedded, hence the term, embedded thunderstorms. contain moderate icing. Altocumulus clouds, which usually form when altostratus clouds are breaking apart, also may To pilots, the is perhaps the most contain light turbulence and icing. dangerous cloud type. It appears individually or in groups and is known as either an air mass or orographic . High clouds form above 20,000 feet AGL and usually form Heating of the air near the Earth’s surface creates an air mass only in stable air. They are made up of ice crystals and pose thunderstorm; the upslope motion of air in the mountainous no real threat of turbulence or aircraft icing. Typical high regions causes orographic thunderstorms. Cumulonimbus level clouds are cirrus, cirrostratus, and cirrocumulus. clouds that form in a continuous line are nonfrontal bands of thunderstorms or lines. Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or Since rising air currents cause cumulonimbus clouds, they cumulonimbus clouds. The bases of these clouds form in are extremely turbulent and pose a significant hazard to flight the low to middle cloud base region but can extend into high safety. For example, if an aircraft enters a thunderstorm, altitude cloud levels. Towering cumulus clouds indicate areas the aircraft could experience updrafts and downdrafts that of instability in the atmosphere, and the air around and inside exceed 3,000 fpm. In addition, thunderstorms can produce them is turbulent. These types of clouds often develop into large hailstones, damaging lightning, tornadoes, and large cumulonimbus clouds or thunderstorms. Cumulonimbus quantities of water, all of which are potentially hazardous clouds contain large amounts of moisture and unstable air to aircraft.

12-16 Cloud classification can be further broken down into specific is classified as very small water droplets, smaller cloud types according to the outward appearance and cloud than 0.02 inches in diameter. Drizzle usually accompanies composition. Knowing these terms can help a pilot identify fog or low stratus clouds. Water droplets of larger size are visible clouds. referred to as rain. Rain that falls through the atmosphere but evaporates prior to striking the ground is known as virga. The following is a list of cloud classifications: and occur when the temperature • Cumulus—heaped or piled clouds of the surface is below freezing; the rain freezes on contact with the cooler surface. • Stratus—formed in layers • Cirrus—ringlets, fibrous clouds, also high level clouds If rain falls through a temperature inversion, it may freeze above 20,000 feet as it passes through the underlying cold air and fall to the ground in the form of . Ice pellets are an indication • —common base with separate vertical of a temperature inversion and that freezing rain exists at a development, castle-like higher altitude. In the case of hail, freezing water droplets are • Lenticularus—lens-shaped, formed over mountains carried up and down by drafts inside cumulonimbus clouds, in strong winds growing larger in size as they come in contact with more • Nimbus—rain-bearing clouds moisture. Once the updrafts can no longer hold the freezing water, it falls to the Earth in the form of hail. Hail can be • Fracto—ragged or broken pea sized, or it can grow as large as five inches in diameter, • Alto—middle level clouds existing at 5,000 to 20,000 larger than a softball. feet Snow is precipitation in the form of ice crystals that falls Ceiling at a steady rate or in snow showers that begin, change in For aviation purposes, a ceiling is the lowest layer of clouds intensity, and end rapidly. Snow also varies in size, from very reported as being broken or overcast, or the vertical visibility small grains to large flakes. are the equivalent into an obscuration like fog or haze. Clouds are reported of drizzle in size. as broken when five-eighths to seven-eighths of the is covered with clouds. Overcast means the entire sky is covered Precipitation in any form poses a threat to safety of flight. with clouds. Current ceiling information is reported by the Often, precipitation is accompanied by low ceilings and aviation routine weather report (METAR) and automated reduced visibility. Aircraft that have ice, snow, or frost on weather stations of various types. their surfaces must be carefully cleaned prior to beginning a flight because of the possible airflow disruption and Visibility loss of lift. Rain can contribute to water in the fuel tanks. Closely related to and reported ceilings is Precipitation can create hazards on the runway surface itself, visibility information. Visibility refers to the greatest making takeoffs and landings difficult, if not impossible, horizontal distance at which prominent objects can be due to snow, ice, or pooling water and very slick surfaces. viewed with the naked . Current visibility is also reported in METAR and other aviation weather reports, as well as Air Masses by automated weather systems. Visibility information, as Air masses are classified according to the regions where predicted by , is available for a pilot during a they originate. They are large bodies of air that take on the preflight weather briefing. characteristics of the surrounding area or source region. A source region is typically an area in which the air remains Precipitation relatively stagnant for a period of days or longer. During Precipitation refers to any type of water particles that this time of stagnation, the air mass takes on the temperature form in the atmosphere and fall to the ground. It has a and moisture characteristics of the source region. Areas of profound impact on flight safety. Depending on the form of stagnation can be found in polar regions, tropical oceans, and precipitation, it can reduce visibility, create icing situations, dry . Air masses are generally identified as polar or and affect landing and takeoff performance of an aircraft. tropical based on temperature characteristics and maritime or continental based on moisture content. Precipitation occurs because water or ice particles in clouds grow in size until the atmosphere can no longer support A continental polar air mass forms over a polar region and them. It can occur in several forms as it falls toward the brings cool, dry air with it. Maritime tropical air masses form Earth, including drizzle, rain, ice pellets, hail, snow, and ice.

12-17 over warm tropical like the and bring There are four types of fronts that are named according to the warm, moist air. As the air mass moves from its source region temperature of the advancing air relative to the temperature and passes over land or water, the air mass is subjected to of the air it is replacing: [Figure 12-24] the varying conditions of the land or water which modify the • Warm nature of the air mass. [Figure 12-23] • Cold An air mass passing over a warmer surface is warmed from • Stationary below, and convective currents form, causing the air to rise. • Occluded This creates an unstable air mass with good surface visibility. Moist, unstable air causes cumulus clouds, showers, and Any discussion of frontal systems must be tempered with turbulence to form. the knowledge that no two fronts are the same. However, generalized weather conditions are associated with a specific Conversely, an air mass passing over a colder surface does not type of front that helps identify the front. form convective currents but instead creates a stable air mass with poor surface visibility. The poor surface visibility is due to the fact that smoke, dust, and other particles cannot rise A warm front occurs when a warm mass of air advances and out of the air mass and are instead trapped near the surface. replaces a body of colder air. Warm fronts move slowly, A stable air mass can produce low stratus clouds and fog. typically 10 to 25 miles per hour (mph). The slope of the advancing front slides over the top of the cooler air and Fronts gradually pushes it out of the area. Warm fronts contain As an air mass moves across bodies of water and land, it warm air that often has very high humidity. As the warm eventually comes in contact with another air mass with air is lifted, the temperature drops and condensation occurs. different characteristics. The boundary layer between two types of air masses is known as a front. An approaching Generally, prior to the passage of a warm front, cirriform front of any type always means changes to the weather or stratiform clouds, along with fog, can be expected to are imminent. form along the frontal boundary. In the summer months, cumulonimbus clouds (thunderstorms) are likely to develop.

A cP

mP

mP

mT

mT cT mT

Figure 12-23. North American air mass source regions.

12-18 steadily from the of relatively warmer air. For the most Symbols for surface fronts and other significant lines shown on the surface analysis chart part, the dew point remains steady and the pressure levels off. After the passage of a warm front, stratocumulus clouds Warm front red predominate and rain showers are possible. The visibility blue eventually improves, but hazy conditions may exist for a short period after passage. The wind blows from the south- red/blue southwest. With warming temperatures, the dew point purple rises and then levels off. There is generally a slight rise in barometric pressure, followed by a decrease of barometric pressure. Flight Toward an Approaching Warm Front Common chart symbology to depict Figure 12-24. By studying a typical warm front, much can be learned location. about the general patterns and atmospheric conditions that exist when a warm front is encountered in flight. Light to moderate precipitation is probable, usually in the Figure 12-25 depicts a warm front advancing eastward from form of rain, sleet, snow, or drizzle, accentuated by poor St. Louis, Missouri, toward Pittsburgh, Pennsylvania during visibility. The wind blows from the south-southeast, and the a flight from Pittsburgh to St. Louis. outside temperature is cool or cold with an increasing dew point. Finally, as the warm front approaches, the barometric At the time of departure from Pittsburgh, the weather is good pressure continues to fall until the front passes completely. VFR with a scattered layer of cirrus clouds at 15,000 feet. As the flight progresses westward to Columbus and closer During the passage of a warm front, stratiform clouds are to the oncoming warm front, the clouds deepen and become visible and drizzle may be falling. The visibility is generally increasingly stratiform in appearance with a ceiling of 6,000 poor, but improves with variable winds. The temperature rises feet. The visibility decreases to six miles in haze with a falling

CIRRS

CIRROSTRATS

ALTOSTRATS WARM AIR IMBOSTRATS COLD AIR

Indianapolis Columbus Pittsburgh St. Louis 200 miles 400 miles 600 miles

999 1002 1005 1008 1011 1014 1950 21018 1 1017 0010 1818 2960 999 59 068 40 1950 16012 3 65 020 3 56 125 53 166 1 59 26 10 18 10 20 6 65 50 60 34 020 1515 2973 10 St. Louis 1002 Columbus Pittsburgh 1950 13018 6 0060 1410 2990

1950 13012 10 1005 150 1201 3002

1008 1011 1014 1017

Figure 12-25. Warm front cross-section with surface weather chart depiction and associated METAR.

12-19 barometric pressure. Approaching Indianapolis, the weather Fast-Moving Cold Front deteriorates to broken clouds at 2,000 feet with three miles Fast-moving cold fronts are pushed by intense pressure visibility and rain. With the temperature and dew point the systems far behind the actual front. The friction between same, fog is likely to develop. At St. Louis, the sky is overcast the ground and the cold front retards the movement of the with low clouds and drizzle and the visibility is one mile. front and creates a steeper frontal surface. This results in a Beyond Indianapolis, the ceiling and visibility are too low very narrow band of weather, concentrated along the leading to continue VFR. Therefore, it would be wise to remain in edge of the front. If the warm air being overtaken by the Indianapolis until the warm front passes, which may take cold front is relatively stable, overcast and rain may up to two days. occur for some distance behind the front. If the warm air is unstable, scattered thunderstorms and rain showers may Cold Front form. A continuous line of thunderstorms, or , A cold front occurs when a mass of cold, dense, and stable may form along or ahead of the front. Squall lines present air advances and replaces a body of warmer air. a serious hazard to pilots as squall-type thunderstorms are intense and move quickly. Behind a fast-moving cold front, Cold fronts move more rapidly than warm fronts, progressing the skies usually clear rapidly, and the front leaves behind at a rate of 25 to 30 mph. However, extreme cold fronts gusty, turbulent winds and colder temperatures. have been recorded moving at speeds of up to 60 mph. A typical cold front moves in a manner opposite that of a Flight Toward an Approaching Cold Front warm front. It is so dense, it stays close to the ground and Like warm fronts, not all cold fronts are the same. Examining acts like a snowplow, sliding under the warmer air and a flight toward an approaching cold front, pilots can get a forcing the less dense air aloft. The rapidly ascending air better understanding of the type of conditions that can be causes the temperature to decrease suddenly, forcing the encountered in flight. Figure 12-26 depicts a flight from creation of clouds. The type of clouds that form depends Pittsburgh, Pennsylvania, toward St. Louis, Missouri. on the stability of the warmer air mass. A cold front in the Northern Hemisphere is normally oriented in a northeast to At the time of departure from Pittsburgh, the weather is VFR southwest manner and can be several hundred miles long, with three miles visibility in smoke and a scattered layer of encompassing a large area of land. clouds at 3,500 feet. As the flight progresses westward to Columbus and closer to the oncoming cold front, the clouds Prior to the passage of a typical cold front, cirriform or show signs of vertical development with a broken layer at towering cumulus clouds are present, and cumulonimbus 2,500 feet. The visibility is six miles in haze with a falling clouds may develop. Rain showers may also develop due barometric pressure. Approaching Indianapolis, the weather to the rapid development of clouds. A high dew point and has deteriorated to overcast clouds at 1,000 feet and three falling barometric pressure are indicative of imminent cold miles visibility with thunderstorms and heavy rain showers. front passage. At St. Louis, the weather gets better with scattered clouds at 1,000 feet and a ten mile visibility. As the cold front passes, towering cumulus or cumulonimbus clouds continue to dominate the sky. Depending on the A pilot using sound judgment based on the knowledge of intensity of the cold front, heavy rain showers form and may frontal conditions will likely remain in Indianapolis until the be accompanied by lightning, , and/or hail. More front has passed. Trying to fly below a line of thunderstorms severe cold fronts can also produce tornadoes. During cold or a squall line is hazardous, and flight over the top of or front passage, the visibility is poor with winds variable and around the storm is not an option. Thunderstorms can extend gusty, and the temperature and dew point drop rapidly. A up to well over the capability of small airplanes and can quickly falling barometric pressure bottoms out during frontal extend in a line for 300 to 500 miles. passage, then begins a gradual increase. Comparison of Cold and Warm Fronts After frontal passage, the towering cumulus and Warm fronts and cold fronts are very different in nature as are cumulonimbus clouds begin to dissipate to cumulus clouds the hazards associated with each front. They vary in speed, with a corresponding decrease in the precipitation. Good composition, weather phenomenon, and prediction. Cold fronts, visibility eventually prevails with the winds from the west- which move at 20 to 35 mph, travel faster than warm fronts, northwest. Temperatures remain cooler and the barometric which move at only 10 to 25 mph. Cold fronts also possess a pressure continues to rise.

12-20 C O L D A WARM AIR IR

CMLOIMBS

Indianapolis Columbus Pittsburgh St. Louis 200 miles 400 miles 600 miles

1008 1005 1005 1008 1011 1950 30018 10 010 0802 2979

74 071 3 1011 46 4 77 102 1950 20024 3 066 71 75 122 10 42 6 8 33 10 73 3 12 70 010 2423 2974 25 10 35 St. Louis Indianapolis Columbus Pittsburgh 1950 20012 6 1014 025 2524 2983

1950 20012 3 035 2422 2989 1011 1011 1014

Figure 12-26. Cold front cross-section with surface weather chart depiction and associated METAR. steeper frontal slope. Violent weather activity is associated with Stationary Front cold fronts, and the weather usually occurs along the frontal When the forces of two air masses are relatively equal, the boundary, not in advance. However, squall lines can form boundary or front that separates them remains stationary and during the summer months as far as 200 miles in advance of influences the local weather for days. This front is called a a strong cold front. Whereas warm fronts bring low ceilings, stationary front. The weather associated with a stationary poor visibility, and rain, cold fronts bring sudden storms, gusty front is typically a mixture that can be found in both warm winds, turbulence, and sometimes hail or tornadoes. and cold fronts.

Cold fronts are fast approaching with little or no warning, Occluded Front and they bring about a complete weather change in just a An occluded front occurs when a fast-moving cold front few hours. The weather clears rapidly after passage and drier catches up with a slow-moving warm front. As the occluded air with unlimited prevail. Warm fronts, on the front approaches, warm front weather prevails but is other hand, provide advance warning of their approach and immediately followed by cold front weather. There are two can take days to pass through a region. types of occluded fronts that can occur, and the temperatures of the colliding frontal systems play a large part in defining Wind Shifts the type of front and the resulting weather. A cold front Wind around a high-pressure system rotates clockwise, while occlusion occurs when a fast moving cold front is colder low-pressure winds rotate counter-clockwise. When two than the air ahead of the slow moving warm front. When high pressure systems are adjacent, the winds are almost in this occurs, the cold air replaces the cool air and forces the direct opposition to each other at the point of contact. Fronts warm front aloft into the atmosphere. Typically, the cold are the boundaries between two areas of high pressure, and front occlusion creates a mixture of weather found in both therefore, wind shifts are continually occurring within a front. warm and cold fronts, providing the air is relatively stable. Shifting wind direction is most pronounced in conjunction A warm front occlusion occurs when the air ahead of the with cold fronts.

12-21 warm front is colder than the air of the cold front. When this and instability are present, the clouds continue to increase is the case, the cold front rides up and over the warm front. If in vertical height. Continuous, strong updrafts prohibit the air forced aloft by the warm front occlusion is unstable, moisture from falling. Within approximately 15 minutes, the weather is more severe than the weather found in a cold the thunderstorm reaches the mature stage, which is the most front occlusion. Embedded thunderstorms, rain, and fog are violent time period of the thunderstorm’s life cycle. At this likely to occur. point, drops of moisture, whether rain or ice, are too heavy for the cloud to support and begin falling in the form of rain Figure 12-27 depicts a cross-section of a typical cold or hail. This creates a downward motion of the air. Warm, front occlusion. The warm front slopes over the prevailing rising air; cool, precipitation-induced descending air; and cooler air and produces the warm front type weather. Prior violent turbulence all exist within and near the cloud. Below to the passage of the typical occluded front, cirriform and the cloud, the down-rushing air increases surface winds and stratiform clouds prevail, light to heavy precipitation falls, decreases the temperature. Once the vertical motion near the visibility is poor, dew point is steady, and barometric pressure top of the cloud slows down, the top of the cloud spreads drops. During the passage of the front, nimbostratus and out and takes on an -like shape. At this point, the storm cumulonimbus clouds predominate, and towering cumulus enters the dissipating stage. This is when the downdrafts clouds may also form. Light to heavy precipitation falls, spread out and replace the updrafts needed to sustain the visibility is poor, winds are variable, and the barometric storm. [Figure 12-28] pressure levels off. After the passage of the front, nimbostratus and altostratus clouds are visible, precipitation decreases, and It is impossible to fly over thunderstorms in light aircraft. visibility improves. Severe thunderstorms can punch through the tropopause and reach staggering heights of 50,000 to 60,000 feet depending Thunderstorms on latitude. Flying under thunderstorms can subject aircraft A thunderstorm makes its way through three distinct stages to rain, hail, damaging lightning, and violent turbulence. before dissipating. It begins with the cumulus stage, in A good rule of thumb is to circumnavigate thunderstorms which lifting action of the air begins. If sufficient moisture identified as severe or giving an extreme echo by at

CIRRS WARM AIR CMLOIMBS CIRROSTRATS

ALTOSTRATS

C IMBOSTRATS OLD AIR COLD AIR

Indianapolis Columbus Pittsburgh St. Louis 200 miles 400 miles 600 miles

1006 1005 1002 999 999 1002 1005 1006 1011 1014 1017 1020 1950 3102340 8 035 0503 2976

1950 2902845 12 1023 058 005 1816 2970 52 142 47 200 42 076 66 8 1 2 34 7 20 8 32 2 51 40 62 26 2 1950 16017 2 Indianapolis St. Louis Columbus Pittsburgh 080 1110 2970

1950 13012 75 130 0804 3012 1011 1014 1017 1020 1023

Figure 12-27. Occluded front cross-section with a weather chart depiction and associated METAR.

12-22 Cumulus Stage 35 mile height Mature Stage 510 mile height Dissipating Stage 57 mile height

40000 ft.

Euilibrium level

30000 ft.

20000 ft.

0 C 32 F

10000 ft.

5000 ft.

Figure 12-28. Life cycle of a thunderstorm.

least 20 nautical miles (NM) since hail may fall for miles Squall Line outside of the clouds. If flying around a thunderstorm is not A squall line is a narrow band of active thunderstorms. Often an option, stay on the ground until it passes. it develops on or ahead of a cold front in moist, unstable air, but it may develop in unstable air far removed from For a thunderstorm to form, the air must have sufficient water any front. The line may be too long to detour easily and too vapor, an unstable lapse rate, and an initial lifting action to wide and severe to penetrate. It often contains steady-state start the storm process. Some storms occur at random in thunderstorms and presents the single most intense weather unstable air, last for only an hour or two, and produce only hazard to aircraft. It usually forms rapidly, generally reaching moderate wind gusts and rainfall. These are known as air maximum intensity during the late afternoon and the first mass thunderstorms and are generally a result of surface few hours of darkness. heating. Steady-state thunderstorms are associated with weather systems. Fronts, converging winds, and troughs Tornadoes aloft force upward motion spawning these storms that often The most violent thunderstorms draw air into their cloud form into squall lines. In the mature stage, updrafts become bases with great vigor. If the incoming air has any initial stronger and last much longer than in air mass storms, hence rotating motion, it often forms an extremely concentrated the name steady state. [Figure 12-29] from the surface well into the cloud. Meteorologists

have estimated that wind in such a vortex can exceed 200 Knowledge of thunderstorms and the hazards associated with knots with pressure inside the vortex quite low. The strong them is critical to the safety of flight. winds gather dust and and the low pressure generates a funnel-shaped cloud extending downward from the Hazards cumulonimbus base. If the cloud does not reach the surface, All thunderstorms have conditions that are a hazard to aviation. it is a ; if it touches a land surface, it is a ; These hazards occur in numerous combinations. While not and if it touches water, it is a “.” every thunderstorm contains all hazards, it is not possible to visually determine which hazards a thunderstorm contains.

12-23 Turbulence

Anvil

Storm movement

Roll cloud

Wind shear turbulence Wind shear turbulance

First gust Dust

Figure 12-29. Movement and turbulence of a maturing thunderstorm.

Tornadoes occur with both isolated and squall line of an approaching storm. Advisory Circular (AC) 00-54, Pilot thunderstorms. Reports for forecasts of tornadoes indicate Windshear Guide, explains gust front hazards associated with that atmospheric conditions are favorable for violent thunderstorms. Figure 2 in the AC shows a of a turbulence. An aircraft entering a tornado vortex is almost mature stage thunderstorm with a gust front area where very certain to suffer loss of control and structural damage. Since serious turbulence may be encountered. the vortex extends well into the cloud, any pilot inadvertently caught on instruments in a severe thunderstorm could Icing encounter a hidden vortex. Updrafts in a thunderstorm support abundant liquid water with relatively large droplet sizes. When carried above Families of tornadoes have been observed as appendages of the , the water becomes supercooled. When the main cloud extending several miles outward from the area temperature in the upward current cools to about –15 °C, of lightning and precipitation. Thus, any cloud connected to much of the remaining water vapor sublimates as ice crystals. a severe thunderstorm carries a threat of violence. Above this level, at lower temperatures, the amount of supercooled water decreases. Turbulence Potentially hazardous turbulence is present in all Supercooled water freezes on impact with an aircraft. Clear thunderstorms, and a severe thunderstorm can destroy an icing can occur at any altitude above the freezing level, but at aircraft. Strongest turbulence within the cloud occurs with high levels, icing from smaller droplets may be rime or mixed shear between updrafts and downdrafts. Outside the cloud, rime and . The abundance of large, supercooled shear turbulence has been encountered several thousand feet water droplets makes clear icing very rapid between 0 °C and above and 20 miles laterally from a severe storm. A low-level –15 °C and encounters can be frequent in a cluster of cells. turbulent area is the shear zone associated with the gust front. Thunderstorm icing can be extremely hazardous. Often, a “roll cloud” on the leading edge of a storm marks the top of the eddies in this shear, and it signifies an extremely Thunderstorms are not the only area where pilots could turbulent zone. Gust fronts often move far ahead (up to 15 encounter icing conditions. Pilots should be alert for icing miles) of associated precipitation. The gust front causes a anytime the temperature approaches 0 °C and visible moisture rapid, and sometimes drastic, change in surface wind ahead is present.

12-24 Hail Engine Water Ingestion Hail competes with turbulence as the greatest thunderstorm Turbine engines have a limit on the amount of water they hazard to aircraft. Supercooled drops above the freezing level can ingest. Updrafts are present in many thunderstorms, begin to freeze. Once a drop has frozen, other drops latch on particularly those in the developing stages. If the updraft and freeze to it, so the hailstone grows—sometimes into a velocity in the thunderstorm approaches or exceeds the huge ice ball. Large hail occurs with severe thunderstorms terminal velocity of the falling raindrops, very high with strong updrafts that have built to great heights. of water may occur. It is possible that these Eventually, the hailstones fall, possibly some distance from concentrations can be in excess of the quantity of water the storm core. Hail may be encountered in clear air several turbine engines are designed to ingest. Therefore, severe miles from thunderstorm clouds. thunderstorms may contain areas of high water , which could result in flameout and/or structural failure of As hailstones fall through air whose temperature is above 0 one or more engines. °C, they begin to melt and precipitation may reach the ground as either hail or rain. Rain at the surface does not mean the Chapter Summary absence of hail aloft. Possible hail should be anticipated Knowledge of the atmosphere and the forces acting within with any thunderstorm, especially beneath the anvil of a it to create weather is essential to understand how weather large cumulonimbus. Hailstones larger than one-half inch affects a flight. By understanding basic weather theories, a in diameter can significantly damage an aircraft in a few pilot can make sound decisions during flight planning after seconds. receiving weather briefings. For additional information on the topics discussed in this chapter, see the following publications Ceiling and Visibility as amended: AC 00-6, Aviation Weather For Pilots and Flight Generally, visibility is near zero within a thunderstorm Operations Personnel; AC 00‑24, Thunderstorms; AC 00-45, cloud. Ceiling and visibility also may be restricted in Aviation Weather Services; AC 91-74, Pilot Guide: Flight in precipitation and dust between the cloud base and the ground. Icing Conditions; and chapter 7, section 2 of the Aeronautical The restrictions create the same problem as all ceiling and Information Manual (AIM). visibility restrictions; but the hazards are multiplied when associated with the other thunderstorm hazards of turbulence, hail, and lightning.

Effect on Altimeters Pressure usually falls rapidly with the approach of a thunderstorm, rises sharply with the onset of the first gust and arrival of the cold downdraft and heavy rain showers, and then falls back to normal as the storm moves on. This cycle of pressure change may occur in 15 minutes. If the pilot does not receive a corrected altimeter setting, the altimeter may be more than 100 feet in error.

Lightning A can puncture the skin of an aircraft and damage communications and electronic navigational equipment. Although lightning has been suspected of igniting fuel and causing an explosion, serious due to lightning strikes are rare. Nearby lightning can blind the pilot, rendering him or her momentarily unable to navigate either by instrument or by visual reference. Nearby lightning can also induce permanent errors in the magnetic compass. Lightning discharges, even distant ones, can disrupt radio communications on low and medium frequencies. Though lightning intensity and frequency have no simple relationship to other storm parameters, severe storms, as a rule, have a high frequency of lightning.

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